The present invention is concerned with gaskets, in particular, with spirally wound gaskets having a sealing layer with enhanced properties which is based upon chemically exfoliated vermiculite.
Exfoliated vermiculite is a known heat-resistant resilient material. Exfoliated vermiculite is conventionally formed by expanding mineral vermiculite using gas, this material being referred to herein as “gas-exfoliated vermiculite”. The gas may be thermally generated, in which case the product is called “thermally-exfoliated vermiculite” (TEV). TEV may be made by flash-heating mineral vermiculite to 750–1000° C., at which temperature the water (free and combined) in the ore vaporises rapidly and ionic repulsion forces apart the silicate sheets which form the raw material, so bringing about an expansion of 10–20 times perpendicular to the plane of the sheets. The granules formed have a chemical composition which (apart from the loss of water) is virtually identical to that of the raw material. Gas-exfoliated vermiculite may also be made by treating raw vermiculite with a liquid chemical, eg hydrogen peroxide, that penetrates between the silicate sheets and subsequently evolves a gas, eg oxygen, to bring about exfoliation.
A different form of exfoliated vermiculite is known as “chemically-exfoliated vermiculite” (CEV) and is formed by treating the ore and swelling it in water. In one possible preparation method, the ore is treated with saturated sodium chloride solution to exchange magnesium ions for sodium ions, and then with n-butyl ammonium chloride to replace sodium ions with n-C4—H9NH3 ions. On washing with water swelling takes place. The swollen material is then subjected to high shear to produce an aqueous suspension of very fine (diameter below 50 μm) vermiculite particles.
It is known to utilise exfoliated vermiculite as a layer of a sheet gasket, eg an automotive exhaust gasket, and for other purposes. For example, GB 2 193 953 B discloses forming sheet-like gaskets formed from particles of gas-exfoliated vermiculite. Because such particles do not cohere well, they are bound together by fine particles of CEV. The use of CEV as a binder retains heat resistance and resilience, whereas the use of other inorganic binders could result in an incompressible structure. However, although exfoliated vermiculite has excellent heat resistance and a high degree of resilience, it has poor water resistance. Furthermore, such products were manufactured using CEV with a high water content at low solids content and considerable drying problems are encountered during production due to the tendency of CEV containing materials to form a surface skin which prevents the further escape of moisture.
Spirally wound gaskets are well-known and are formed from a metal supporting strip, conventionally of steel, and a sealing strip formed from a resilient material, conventionally expanded graphite (also called exfoliated graphite). In the formation of conventional spirally wound gaskets, the steel supporting strip is fed onto a mandrel. The steel supporting strip is welded either to itself to form a closed loop around the mandrel or, alternatively, is welded to an inner ring of the gasket which is itself mounted on the mandrel. The mandrel is then rotated to draw further supporting strip on to the mandrel to form a planar spiral. Simultaneously, the sealing strip is drawn between the coils of the steel strip so that a spiral of the sealing strip is formed interposed between the coils of the supporting strip. When the gasket spiral has been completed, the steel supporting strip is welded to itself to form a closed loop at the outside of the gasket and the gasket is removed from the mandrel. Such gaskets are utilised, for example, for forming seals between flanges at the ends of pipes. The supporting strip holds the sealing strip in position and the sealing strip forms a seal between the flanges and between the coils of the supporting strip.
It should be clear, from the above description of how spirally wound gaskets are formed that, the sealing strip thereof must have sufficient strength and flexibility to enable it to be drawn into the spiral and formed into a gasket without breakage. A sealing strip formed from expanded graphite foil, although relatively brittle, does have sufficient strength.
In many cases, it is desirable for a spirally wound gasket to have a high degree of heat resistance but, in a conventional gasket, the heat resistance is limited by that of the expanded graphite which is lower than is desirable.
It is a further object of at least some of the preferred embodiments of the present invention to provide a spirally wound gasket in which the sealing strip has increased heat resistance.
WO98/53022 discloses the possibility of a spirally wound gasket having a sealing strip produced from CEV compositions with a carrier strip to prevent breakage of the resilient material which otherwise is deemed to be too brittle to allow formation of a spirally wound gasket. This technique has encountered problems with delamination of the carrier strip during slitting of the resilient layer. As a result, adhesive has been applied in some instances to the resilient layer to prevent delamination from the carrier strip.
Unfortunately, providing adhesive increases the organic content in the resilient layer and organic components tend to burn out at elevated temperatures causing voids to form which provide leakage pathways through the material, shrinkage and stress relaxation.
Furthermore, use of a carrier strip increases processing cost and complexity. As the use of graphite with carrier strips does not provide leakage problems, ways of applying the carrier strip technology to brittle vermiculite materials have been on-going but without effective solutions to date.
It is a further object of at least some of the preferred embodiments of the present invention to provide a spirally wound gasket comprising a sealing layer with improved water resistance. It is a further such object to provide a spirally wound gasket with a sealing layer with reduced loss in stress retention and low creep. It is a further such object of the present invention to provide a spirally wound gasket with surprising improvements.